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Recent advancement in personal communication services (PCS) provides wireless multimedia services for users on the move. For establishing the convenient access environment, the evolution of IP-based integrated scheme becomes very urgent for public wireless access systems. In this paper, we clarify the well-known issues of integrating the general packet radio service (GPRS) network and IEEE 802.11 wireless local area network (WLAN). These issues include the inconsistence in transmission rate and coverage area, the difficulty in seamless handoff, the complexities of mobile IP and address translations, and so on. Based on classified cases, we propose the circular probe strategy (CPS) to measure the precise handoff latency of a mobile node roaming from one network to another. This information helps mobile nodes and its home agent (HA) to decide the appropriate handoff timing, to maximize the data transmission rate and to perform seamless handoff in the heterogeneous system.